An anti-A agglutinin has been found in an seed extract of Falcata japonica. In titration of the extract the A1 cells are more strongly agglutinated than the A2 cells. Five hundred O and B red cell samples including OWk+ are not agglutinated with Falcata extract. The extract agglutinates much more strongly with papain treated A red cells, and papain treated O and B red cells come to be agglutinated with the extracts. Absorption with papain treated O red cells leaves an anti-A agglutinin reacts with papain treated A red cells. The agglutination reactions of extract of Falcata japonica with untreated A, papain treated A and O red cells are specifically inhibited by A secretor saliva. As simple sugars, N-acetyl-D-galactosamine is a potent inhibitor of the reactions.